1. Field of the Invention
The present invention relates to an X-ray optical device and a multilayer mirror for use in small angle scattering of a material, to converge X-rays radiated from an X-ray source on a sample.
2. Description of the Related Art
Depending on a material, X-rays are sometimes scattered in a small angle region (for instance, around 0xc2x0 to 5xc2x0) with an optical axis of incident X-rays at the center, when X-rays are irradiated.
For instance, when a material has fine particles of about 10 to 1000 angstroms or a region of inhomogeneous density in the equivalent size thereto, scattering which is diffused in an incident direction of X-rays, the so-called small angle scattering, occurs. The small angle scattering gets broader as particles become smaller, regardless of an inner structure of the particles. This scattering exits without being dependent on whether a material is crystalline and amorphous. The scattering is observed at a scattering angle, in other words, in a small angle region where an angle from an optical axis of incident X-rays is about 0xc2x0 to 5xc2x0.
For a small angle region, the diffraction when a lattice spacing is extremely large like in protein crystals, X-ray diffraction in case of the so-called long-period structure in which crystalline and amorphous are periodically arranged in a fiber sample, and so forth are observed in a small angle region.
X-rays observed in a small angle region, including the diffuse scattering, and diffraction, are generally called small angle scattering. By the small angle scattering, various characteristics of a sample may be determined.
For the small angle scattering, a total reflection mirror having a cylinder face is conventionally used for an optical system to converge X-rays on a sample. In a convergent optical system using this type of a total reflection mirror, a distance from the total reflection mirror as the center to an X-ray source (X-ray focal point) is equal to a distance from the mirror to a convergent point of X-rays reflected at the total reflection mirror.
As the distance from the total reflection mirror to a convergent point of X-rays increases, a convergent angle of x-rays becomes smaller. Accordingly, small angle resolution improves, which is preferable.
However, when the distance from the total reflection mirror to a convergent point of X-rays is widened to improve small angle resolution, the distance from an X-ray source (X-ray focal point) to the total reflection mirror also increases in proportion to the distance. Additionally, since X-rays attenuate more as the X-ray source is set farther away from the total reflection mirror, the intensity of incident X-rays to a sample declines.
Thus, the conventional optical system for use in small angle scattering has a problem in that either small angle resolution or intensity of incident X-rays to a sample is sacrificed.
In order to solve this problem, an optical system using a special mirror, called a toroidal mirror, was considered. However, this toroidal mirror having a complex shape is extremely difficult to manufacture, and is impractical.
Accordingly, it is an object of the present invention to optimize both small angle resolution and intensity of incident X-rays to a sample in small angle scattering.
In order to achieve the above object, an X-ray optical device for small angle scattering system relating to the present invention includes a multilayer mirror providing a divergent angle xcex4 of reflected X-rays, and an X-ray source to radiate X-rays. The multilayer mirror has at least the following (a) and (b) conditions:
(a) reflection faces are elliptic having two focal points A and B; and
(b) a distance from the center of the reflection faces to the focal point B (convergent point of reflected X-rays) is set to provide a convergent angle xcex8c of X-rays at the focal point B that is almost twice as great as the divergent angle xcex4, when X-rays are irradiated to the reflection faces by arranging the X-ray source at the focal point A.
With such a configuration, a distance from the center of the reflection faces of the multilayer mirror to the X-ray source becomes short, and the attenuation of X-rays diverging from the X-ray source is inhibited. A sample may be arranged at a random location along the path of X-rays reflected at the multilayer mirror. However, if the sample is arranged at the other focal point B of the multilayer mirror, an irradiation area of X-rays would be extremely small, and small angle scattering would be performed with a small amount of sample.
With a small amount of sample, a capillary column or the like which absorbs little X-rays, may be used as a sample container. Scatter X-rays of higher intensity from the sample may be obtained. Small angle scatter X-rays from the sample are detected by an X-ray detector provided downstream from a convergent point.
When X-rays a are entered to a surface of a multilayer mirror 1 at an incident angle xcex8 as shown in FIG. 2A, X-rays b are reflected at an angle of 2xcex8 with respect to the incident X-rays a. However, due to surface roughness of the multilayer mirror l,etc. X-rays c diverge at a minute angle xcex4 with the reflected X-rays b as the center.
In the present invention, the divergent angle xcex4 of X-rays at the multilayer mirror indicates a divergent angle xcex4 of the reflected X-rays c, and is equivalent to xc2xd of a full width of a rocking curve. However, the definition of the full width of a rocking curve is difficult to make, and is not standardized. In the present invention, it is defined that the full width of a rocking curve is twice as wide as a full width at half maximum of the peak of the rocking curve. The divergent angle xcex4 is assumed to be the full width at half maximum of the peak. When a standardized definition is established for the full width of a rocking curve, the divergent angle xcex4 of the present invention may be derived on the basis of this definition.
A full width at half maximum of the peak of a rocking curve may be derived as follows. Specifically, as shown in FIG. 2B, the X-rays a are entered to a surface of the multilayer mirror 1 from an X-ray source 2 at the incident angle xcex8, and an X-ray detector 3 is arranged at an angle of 2xcex8 with respect to the incident X-rays a. When the multilayer mirror 1 is xcfx89-rotated only by a minute angle from this state, an X-ray profile I may be obtained as shown in FIG. 2C. The X-ray profile I is called a rocking curve. A width xcex4 at xc2xd of the peak intensity thereof is a full width at half maximum of the peak of the rocking curve.
The convergent angle xcex8c of the multilayer mirror indicates a convergent angle of a whole X-ray flux that is reflected at the mirror and is converged on one point. Since the divergent angle xcex4 of X-rays is minute at the multilayer mirror, the convergent angle xcex8c becomes small even if the convergent angle xcex8c is twice as great as the divergent angle xcex4. Accordingly, reflected X-rays at the downstream from the convergent point diverge in a small angle.
Furthermore, since the convergent angle xcex8c of the multilayer mirror is set as above, almost parallel loci are formed by the reflected X-rays c that appear at the divergent angle xcex4 (≈xcex8c/2) around the reflected X-rays b converging at the convergent angle xcex8c as shown in FIG. 3.
Accordingly, an X-ray detector may be arranged at a location apart from the convergent point of X-rays (in other words, sample location), and high small angle resolution may be obtained.
Moreover, it is preferable in the present invention that a first slit and a second slit are arranged along an optical axis of X-rays that are reflected at the multilayer mirror, to prevent the X-rays from diverging, and a third slit is arranged near another focal point (convergent point of X-rays) of the multilayer mirror to shield parastic scattering from the multilayer mirror.
It is a well-known fact that parastic scattering from a multilayer mirror negatively affects small angle scattering. According to the optical device of the present invention, since a distance between the multilayer mirror and a sample is kept sufficiently long, the effects of parastic scattering are small. Furthermore, by inserting the third slit, parastic X-ray scattering may be shielded better. When a sample is arranged at another focal point (convergent point of X-rays) of the multilayer mirror, the third slit may be arranged near the convergent point. Thus, a slit width may be reduced without attenuating the intensity of X-rays, and parastic scattering may be shielded while maintaining X-ray intensity.
As explained above, according to the optical device for small angle scattering system of the present invention, both small angle resolution and intensity of incident X-rays to a sample may be optimized, and small angle scattering measurement may be performed at high precision.